itcamp 2011 - catalin zima - common pitfalls in windows phone 7 game development

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Common Pitfalls in Windows Phone 7

Game Development

Catalin Zima-Zegreanu Evozon Systems

email: [email protected] blog: http://catalinzima.com

twitter: CatalinZima Microsoft XNA MVP

mailto:[email protected]

http://catalinzima.com/


IT Camp 2011

• Thanks for coming!

• ITCamp is made possible by our sponsors:


Session agenda

• Introduction

• Performance

• Certification

• Devices

• Isolated Storage

• Miscellaneous

• Q&A


INTRODUCTION

What? Who? Why?


WP7 game development

• Windows Phone 7 – One of the three major players in the smartphone

world

– Standard chassis specifications

– 10 models from 4 manufacturers (more coming soon)

– New programming model

• XNA Game Studio 4.0 – One of the technologies available for WP7

development

– Recommended for Games

– Cross platform game development using C#


The questions

• What does this session contain? – Common „gotchas‟ – Tips and advice for avoiding them – NOT an introduction to XNA and WP7

(but don‟t go! This is still useful!)

• Who is the target audience? – Game developers using XNA – Game developers targeting WP7

• Why should you care? – Learn from our mistakes – Repairing is harder than preventing


PERFORMANCE

Where we see all sort of things that can slow down an XNA game


Loading Times

• Users like games that load fast

• Also, the watchdog watches you

– If the game does not show something to the user in 10 seconds, it is closed

• Proper techniques can reduce loading times significantly

• Example: Chickens Can Dream

– From 14 seconds to ~4 seconds


Techniques: Compression

• Use DXT compression for images

– Requires power-of-two size

– Unlike PNG or JPEG, the texture is actually compressed in the GPU memory

– 1/8 compression for opaque images

– ¼ compression for images with transparency

• Precompile XML using the Content Pipeline -> much smaller sizes


Techniques: Spritesheets

• Pack multiple sprites in a single texture – http://create.msdn.com/en-US/education/catalog/sample/sprite_sheet – http://spritesheetpacker.codeplex.com/

• Use sourceRectangle parameter of SpriteBatch.Draw to draw individual sprites

• Benefits: – Faster Loading (one big file instead of multiple small

ones) – Avoids DXT power-of-two restrictions – Bonus: faster drawing (fewer texture switches)

http://create.msdn.com/en-US/education/catalog/sample/sprite_sheet




http://spritesheetpacker.codeplex.com/

http://spritesheetpacker.codeplex.com/


Spritesheet example


DEMO

Loading Times


Techniques: Blame someone else

• Phones without the NoDo update have significantly slower loading times

– Tell everyone to apply the update

• Some HTC phones have loading times much slower than all other phones

– Suffer the wrath of HTC owners, or just go back to the two previous techniques


Garbage Collection

• .Net CF uses a non-generational GC

– At each collection, the whole heap is „swept‟

– Much slower that a generational GC

• If you generate garbage each frame => hiccups

• Mango Update– good news

– .Net CF receives generational GC

– This still doesn‟t solve the problem, just hides it

– Garbage is the #1 enemy for your performance


Garbage Collection

• If you think you have garbage issues, profile and see who‟s allocating – Use a Windows build of the project and the .Net CLR

Profiler – Mango adds more profiling tools for the WP7 build

• Common sources – String manipulations – Linq extension methods – Boxing – foreach – don‟t use it for arrays

• Use Object Pools


Strings

• Strings are immutable

• String manipulation => Garbage creation – .ToString()

– String.Format()

• Use StringBuilder – Mutable

– SpriteBatch.DrawString has an overload that accepts StringBuilder


LINQ Extension methods

• They all create garbage.

– use of delegates

– use of IEnumerable

• Intellisense makes these easy to use by accident.

• Removing “using System.Linq” will prevent accidents


Boxing

• Value types used as reference type

– structs that implement interfaces

– Dictionaries that use Enums as keys

• IEqualityComparer<T>.Default uses object

• See http://bit.ly/nick_enums for solution

– Methods working wit the object class

• Use Generics as often as possible

http://bit.ly/nick_enums

http://bit.ly/nick_enums


Using Object Pools

• Use a class to manage a list of instances for reference types – When you need new instance, instead of using

new, request an instance from the pool – When done with an instance, release it back to the

pool

• Objects are pre-allocated • Management of objects has to be done

manually (back to the dark ages?) • Heap complexity is increased

– Occasional GCs are slower


Other Performance Tips

• Guide.IsTrial – Slow: cache and reuse • Manual Inline sensitive code - Nasty, but efficient • Use public fields instead of Properties

– Please hold the tomatoes back!

• Use „out‟ and „ref‟ methods when dealing with large value types (Matrix, Vector2, Vector3)

• Use jagged arrays instead of 2D arrays • GPU Fillrate - use a smaller resolution to investigate • GPU batching –use spritebatch to batch more Draw

calls together

• Profile, profile, profile!


CERTIFICATION

Where we try to pass certification in the first try


Common certification fails

• Publisher name and version number

• The Back Button

• Background music

• Memory Limits

• Phone capabilities


Publisher name and version number

• Rule 5.6 - Technical Support Information

– An application must include the application name, version information, and technical support contact information that are easily discoverable.

• Before November 1st 2011, this was only „recommended‟

• One of the most common cert. failures

• Starting with June 3, this will return to „recommendation‟ status.


The Back Button

• Rule 5.2.4

• Common failures: back button exits the application

• Why? Default XNA template: protected override void Update(GameTime gameTime) { // Allows the game to exit if (GamePad.GetState(PlayerIndex.One).Buttons.Back == ButtonState.Pressed) this.Exit(); // TODO: Add your update logic here base.Update(gameTime); }


User Music

• To summarize rule 6.5:

– Don‟t mess with the user‟s background music

– Don‟t crash when user is playing music

• Simple solution:

– Use MediaPlayer.GameHasControl


Memory Limits

• Phones have a minimum of 256MB

• Your game has a maximum of 90MB

• The game simply exits without having the chance to do anything.

• Use DeviceExtendedProperties to query memory while debugging and keep usage low

• Don‟t use it too often, as it generates garbage


Phone Capabilities

• Not a Fail reason, but still good to know • During certification, your manifest file gets regenerated

– May result in detecting capabilities you don‟t actually use – Ex: Having a Reference to System.Net => Game uses data

connection

• Users may chose not to install the game based on the shown capabilities

• You just lost a customer

• Solution: Use the Windows Phone Capability Detection Tool before submitting your game


DEVICES

Where we see that not all devices are created equal


Device Discrepancies

• Samsung Phones with AMOLED screens

– Color banding in gradients is more visible

• HTC Devices

– Experience with HD7 and 7 Pro, all reported

– SoundEffectInstance.Volume doesn‟t work

– If application is interrupted without tombstoning (eg. phone call), the Accelerometer class doesn‟t work anymore, and a new instance needs to be created.


Remember the QWERTY

• Common scenario

– In your Windows build, you bind something to keyboard keys (like invincibility, etc)

– You only have non-QWERTY devices to test

– You ship with the „forgotten‟ code in

– Some users discover the „cheat‟

• We shipped with such code in and it made us doubt the users‟ highscores

• Tip: Use compile #if directives for your „cheats‟


ISOLATED STORAGE

Where we see what bad things can happen in isolation


Program Flow

• Game is launched or reactivated

• Load content to show on the screen – To make sure the watchdog doesn‟t get us

• Load saved data from IsolatedStorage

• … game is played …

• On Deactivated is called

• Save user data to IsolatedStorage


Program Flow

• Game is launched or reactivated • Load content to show on the screen • User hits the Back button before loading is

done • Load saved data from IsolatedStorage

• … game is played …

• On Deactivated is called • Save user data (null) to IsolatedStorage


Accidentally deleting data

• Data is overwritten => User settings and data are now gone

• Progress is erased • User is angry -> bad review

• What to do ?

– Check for null before overwriting – Save data directly to disk whenever it changes, not on

Deactivated event – Keep a backup of the save file – Use different files for different data


Game Updates - Scenario

• Use a class, let‟s call it… UserData to store all the user settings and progress

• On save, serialize class to disk using XML

• On load, deserialize from XML

• On first run, if no data exists, create default UserTemplate instance and save it to disk

• All works ok, QA approves, data is saved!


Game Updates - Scenario

• Add new functionality to the game

• New data added to the UserData class

• Added code to the code that properly creates the default instance

• QA tests the new game version, all is well

• Everyone happy, game update is released

• The Nightmare begins!!!


What went wrong?

• Updating user has an older version of the save file in his IsolatedStorage

• Deserialization failes because the class is changed

• QA can‟t test this scenario • Possible changes

– Add new value-type fields to UserData – Add new reference type field to UserData – Change the type of a member of UserData – Remove any member of the UserData


True story, bro!

• Our first update to Chickens Can Dream faced this problem

• We simply added a List<Boolean> • 15.000 users who updated the game now had the

game crashing on startup • Fixed the bug, updated, problem solved!

• However:

– Average ratings dropped by almost a full star – We needed a full month to get back to the ratings before

the update – Lots of reviews in the Marketplace saying that the game

doesn‟t work after the update


How to avoid?

• Be careful – Pray someone doesn‟t miss something

– Check for nulls all over the place

– Use post-its to remind yourself!

• Use versions for your save files – Tied to your game‟s version

– Code that reads an older version and converts to a new version


MISCELLANEOUS

Where we talk about stuff that didn‟t fit anywhere else

User Input

• Unsupported Characters

– You ask for a name from the player

– Player uses non-ASCII characters

– When trying to draw the name => CRASH

• Global Highscores? That‟s just bad luck!

– Crisis averted in CCD by sanitizing text server-side

• Why it happens?

– Fonts are used as images in XNA

– The default SpriteFont only supports ASCII

• Fix: uncomment the DefaultCharacter tag

• (or use Mango and Silverlight/XNA integration)

 <DefaultCharacter>*</DefaultCharacter>


Tombstoning and resources

• There is a tendency to dispose and unload resources when OnDeactivated is called

• DON‟T!

• Currently, for very short periods of time the game might not be evicted from memory => ObjectDisposedException

• After MANGO, it may never be evicted

• TODO: just save your state, don‟t try to clear or dispose anything


Q&A

Challenge Accepted!


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itcamp 2011 - catalin zima - common pitfalls in windows phone 7 game development

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